The present invention relates to a dynamic bearing device and a motor having the same. This bearing device can be appropriately applied in a spindle motor of an information-processing equipment such as a magnetic disk device (e.g., HDD or FDD), an optical disk device (e.g., CD-ROM or DVD-ROM), an optical magnetic device (e.g., MD or MO), a polygon scanner motor of a laser beam printer (LBP), or a small-sized motor of an electric equipment (e.g., an axial flow fan).
Heretofore, each of the motors described above has been in need of improvements on speeding up and noise-reduction of its rotary motion, cost-reduction in its production, and so on in addition to providing a rotary motion thereof with a high degree of precision. One of the structural factors that define these required performances is a bearing that supports a spindle of the motor. In recent years, for such a kind of the bearing, the usage of a dynamic bearing having excellent features for the above required performances has been considered or actually used in the art.
For instance, a dynamic bearing device to be incorporated in a spindle motor of a disk device such as a hard disk drive (HDD) includes a radial bearing part that makes a non-contact support of an shaft member in a rotatable manner in the radial direction and a thrust bearing part that makes a non-contact support of an shaft member in the thrust direction. The dynamic bearing device utilizes a dynamic bearing as each of these bearing parts. The dynamic bearing has grooves for generating dynamic pressure in the bearing surface. Hereinafter, such grooves will be referred to as dynamic pressure generating grooves. The dynamic pressure generating grooves of the radial bearing part are formed in the inner peripheral surface of a housing or a bearing member, or the outer peripheral surface of a shaft member. When an shaft member having a flange part is used, the dynamic pressure generating grooves of the thrust bearing part are formed in both surfaces of the flange part or the surfaces facing to the respective surfaces (e.g., the end surface of the bearing member and the bottom surface of the housing).
As the above dynamic bearing device, the present applicant has already proposed the constitution of a dynamic bearing device 1xe2x80x2 shown in FIG. 7 as disclosed in Japanese Patent Application No. 2001-114317.
In FIG. 7, the dynamic bearing device 1xe2x80x2 mainly includes a bottomed cylindrical housing 7xe2x80x2 with an opening 7axe2x80x2 formed in its top end, an shaft member 2xe2x80x2 and a bearing member 8xe2x80x2 which are housed in the housing 7xe2x80x2, and a sealing member 10, arranged on the opening 7axe2x80x2 of the housing 7xe2x80x2.
More concretely, the housing 7xe2x80x2 includes a cylindrical side portion 7bxe2x80x2 and a bottom part 7cxe2x80x2. Furthermore, the bottom part 7cxe2x80x2 has an inner bottom surface 7c1xe2x80x2. In the area of the bottom surface 7c1xe2x80x2 which serves as a thrust bearing surface, as shown in FIG. 8B, herringbone-shaped dynamic pressure generating grooves 7c2xe2x80x2 are formed.
Furthermore, the bearing member 8xe2x80x2 is constructed of a porous material made of sintered metal. Herringbone-shaped dynamic pressure generating grooves 8a1xe2x80x2 and 8a2xe2x80x2 as shown by the dotted lines in FIG. 7 are formed in the upper and lower areas which serve as radial bearing surfaces, respectively. These upper and lower areas are separated in the axial direction such that an area 8a3xe2x80x2 having no dynamic pressure generating groove is arranged between the upper and lower areas. Furthermore, in an area, which serves as a thrust bearing surface, of the lower end surface 8bxe2x80x2 of the bearing member 8xe2x80x2, herringbone-shaped dynamic pressure generating grooves 8b1xe2x80x2 shown in FIG. 8A are formed.
The shaft member 2xe2x80x2 includes an axial part 2axe2x80x2 and a flange part 2bxe2x80x2. The flange part 2bxe2x80x2 is integrally or separately formed on the axial part 2axe2x80x2. 
The axial part 2axe2x80x2 of the shaft member 2xe2x80x2 is inserted in the inner peripheral surface 8axe2x80x2 of the bearing member 8xe2x80x2. The flange part 2bxe2x80x2 is received in a space between the lower end surface 8bxe2x80x2 of the bearing member 8xe2x80x2 and the inner bottom surface 7c1xe2x80x2 of the housing 7xe2x80x2. Predetermined Radial bearing gaps are formed between the upper and lower areas of the inner peripheral surface 8axe2x80x2 of the bearing member 8xe2x80x2 which serves as radial bearing surfaces and the outer peripheral surface 2a1xe2x80x2 of the axial part 2axe2x80x2, respectively. Predetermined thrust bearing gaps are formed between the area of the lower end surface 8bxe2x80x2 of the bearing member 8xe2x80x2 which serves as the thrust bearing surface and the upper surface 2b1xe2x80x2 of the flange part 2bxe2x80x2, and between the area of the inner bottom surface 7c1xe2x80x2 of the housing 7xe2x80x2 which serves as the thrust bearing surface Sand the lower surface 2b2xe2x80x2 of the flange part 2bxe2x80x2.
The inner space of the housing 7xe2x80x2 being sealed with a sealing member 10xe2x80x2, including pores of the bearing member 8xe2x80x2, is filled with a lubricating oil.
When the shaft member 2xe2x80x2 rotates, a dynamic pressure action of the lubricating oil is generated in the radial bearing gaps, so that the axial part 2axe2x80x2 of the shaft member 2xe2x80x2 is rotatably supported in the radial direction in a non-contact manner by the oil film of the lubricating oil formed in the radial bearing gaps. Thus, the radial bearing parts R1xe2x80x2 and R2xe2x80x2 which rotatably support the axial part 2axe2x80x2 in the radial direction in a non-contact manner are constituted. Simultaneously, a dynamic pressure action is generated in the thrust bearing gaps, so that the flange part 2bxe2x80x2 of the shaft member 2xe2x80x2 is rotatably supported in the thrust directions in a non-contact manner by the oil film of the lubricating oil formed in the thrust bearing gaps. Thus, the thrust bearing parts S1xe2x80x2 and S2xe2x80x2 which rotatably support the flange part 2bxe2x80x2 in the thrust directions in a non-contact manner are constituted.
In the dynamic bearing device 1xe2x80x2 constituted as above, the dynamic pressure generating grooves 8axe2x80x2 and 8a2xe2x80x2 of the radial bearing parts R1xe2x80x2 and R2xe2x80x2 have their respective herringbone shapes which are symmetric with respect to the axial direction. Therefore, in the radial bearing part R1xe2x80x2, the lubricating oil drawn from the both sides in the axial direction by the dynamic pressure generation grooves 8axe2x80x2 keeps its pressure balance at a position in proximity to the axial groove center of the dynamic pressure generating grooves 8a1. Likewise, in the radial bearing part R2xe2x80x2, the lubricating oil drawn from the both sides in the axial direction by the dynamic pressure generating groove 8a2xe2x80x2 keeps its pressure balance at a position in proximity to the axial groove center of the dynamic pressure generating grooves 8a2xe2x80x2. At this time, since the radial bearing surface of each of the radial bearing parts R1xe2x80x2 and R2xe2x80x2 has a plurality of surface openings, which are formed by the pores of the bearing member 8xe2x80x2 opening to the surface, in the radial bearing gap, where the pressure of the lubricating oil increases, the lubricating oil is returned from the radial bearing gap into the inside of the bearing. In addition, since there is a drawing action of each of the dynamic pressure generating grooves 8a1xe2x80x2 and 8a2xe2x80x2, in the peripheral area of the radial bearing gap, the lubricating oil is supplied from the inside of the bearing into the radial bearing gap. The above pressure balance can be kept while being accompanied with such a circulation of the lubricating oil. However, there is a case that the dynamic pressure generating grooves 8a1xe2x80x2 are formed with an asymmetric shape with respect to the axial direction as a result of manufacturing errors. In this case, the axial dimension of the lower groove region is larger than that of the upper groove region in the figure. Also, there is a case that the dynamic pressure generating grooves 8a2xe2x80x2 are formed with an asymmetric shape with respect to the axial direction as a result of manufacturing errors. In this case, the axial dimension of the upper groove region is larger than that of the lower groove region in the figure. In these cases, between the upper and lower regions, there is a difference in the forces of drawing the lubricating oil into their grooves. As a result, the above pressure balance comes down. Therefore, the lubricating oil in the gap of an area between the radial bearing parts R1xe2x80x2 and R2xe2x80x2 (here, this gap is referred to as an X portion as shown by dotted circle in FIG. 7 and is larger than the radial bearing gap) is drawn into the radial bearing part R1xe2x80x2 and/or the radial bearing part R2, causing a negative pressure in the X portion. In addition, there is a case that the radial bearing gap of the radial bearing part R1xe2x80x2 is taper-shaped increasing upwardly as a result of manufacturing errors. Alternatively, there is another case that the radial bearing gap of the radial bearing part R2xe2x80x2 is taper-shaped increasing downwardly as a result of manufacturing errors. In these cases, a flow of the lubricating oil in the radial bearing gap to the increased gap side is generated as the pressure of the increased gap side becomes decreased. As a result, the above pressure balance comes down. Therefore, there is a case that the lubricating oil in the X portion is drawn into the radial bearing part R1xe2x80x2 and/or the radial bearing part R2, causing a negative pressure in the X portion.
In the above constitution of the dynamic bearing device 1xe2x80x2, the dynamic pressure generating grooves 8b1xe2x80x2 and 7c2xe2x80x2 of the thrust bearing parts S1xe2x80x2 and S2xe2x80x2 have their respective herringbone shapes which are symmetric with respect to the radial direction. In the thrust bearing part S1xe2x80x2, the lubricating oil in the thrust bearing gap and in the surroundings thereof is drawn toward its radial groove center by means of the dynamic pressure generating grooves 8b1xe2x80x2. Likewise, in the thrust bearing part S2xe2x80x2, the lubricating oil in the thrust bearing gap and in the surroundings thereof is drawn toward its radial groove center by means of the dynamic pressure generating grooves 7c2xe2x80x2. Therefore, there is a case that a negative pressure may be caused in each of gaps as shown in FIG. 7, that is, a gap around the boundary between the axial part 2axe2x80x2 and the flange part 2bxe2x80x2 (here, this gap is referred to as a Y portion as shown by dotted circle in FIG. 7), a gap at an inner diameter side region than the thrust bearing part Sxe2x80x2 (here, this gap is referred to as a U portion as shown by dotted circle in FIG. 7 and is larger than the gap of the thrust bearing gap), and a gap between the outer peripheral surface of the flange part 2bxe2x80x2 and the inner peripheral surface of the housing 7xe2x80x2 (here, this gap is referred to as a Z portion as shown by dotted circle in FIG. 7 and is larger than the radial bearing gap).
If the negative pressure generated in the inside of the housing 7xe2x80x2 is large, then a cavitation is generated and the air solved in the lubricating oil can be emerged as air bubbles. If such air bubbles are involved in the bearing part, the accuracy of a rotation can be degenerated, so that RRO (Repeatable Run Out) and NRRO (Non Repeatable Run Out) can be deteriorated. Furthermore, if the temperature increases while accompanying with the air bubble, as the expansion of the air bubble, the lubricating oil in the housing 7xe2x80x2 is pushed out of the sealed space between the inner peripheral surface of the sealing member 10xe2x80x2 and the outer peripheral portion of the axial part 2axe2x80x2. As a result, there is a possibility of causing the leak of the lubricating oil to the outside.
An object of the present invention is to prevent the generation of a negative pressure in the housing and a cavitation due to the negative pressure, thereby to increase in a rotational accuracy and a sealing performance against a lubricating oil in a dynamic bearing device and a motor having the same.
To attain the above object, there is provided a dynamic bearing device comprising a housing having an end with an opening and another end with a bottom part, an shaft member having an axial part and a flange part to be housed in the housing, a cylindrical bearing member to be housed in the housing, constituted by a porous body made of a sintered metal, at least one radial bearing part provided between an inner peripheral surface of the bearing member and an outer peripheral surface of the axial part, which supports the axial part in the radial direction in a non-contact manner by means of a dynamic pressure action of a lubricating oil to be caused in a radial bearing gap, thrust bearing parts provided between both surfaces of the flange part and an end surface of the bearing member and the bottom part of the housing, which support the flange part in the thrust directions in a non-contact manner by means of a dynamic pressure action of a lubricating oil to be caused in thrust bearing gaps, and a sealing member arranged in the opening of the housing, wherein an inner space of the housing including pores of the bearing member is filled with a lubricating oil, and wherein the bearing member comprises an area at the outer diameter side of an end surface area of the bearing member which constitutes the thrust bearing gap, the area having a percentage of surface openings of the pores larger than that of the end surface area.
In the above constitution of the dynamic bearing device, the radial bearing part may have dynamic pressure generating grooves which are shaped so that the lubricating oil in the radial bearing gap and its surroundings is drawn to the bottom part side of the housing.
In the above constitution of the dynamic bearing device, the thrust bearing part may have dynamic pressure generating grooves which are in the shape of one selected from a spiral shape by which the lubricating oil in the thrust bearing gap and its surroundings is drawn in the inner diameter direction and a herringbone shape by which the lubricating oil in the thrust bearing gap and its surroundings is drawn toward a radial center portion of the thrust bearing part.
In the above constitution of the dynamic bearing device, the area having the larger percentage of surface openings may be provided on an inclined surface which is formed in a direction that the thrust bearing gap increases at the outer diameter side of the end surface area of the bearing member, for example, a chamfered surface. Alternatively, the area having the larger percentage of surface openings may be provided by a recessed portion which is formed in the outer peripheral portion of the bearing member and continuous to the end surface area of the bearing member. Such a recessed portion may be also continuous to the end surface area of the bearing member at he opening side of the housing, however, it is preferable that the recessed portion is not continuous to the end surface area at the opening side. When the recessed portion is also continuous to the end surface area at the opening side, there is a possibility that the effects intended in the present invention cannot be obtained in a sufficient manner because the lubricating oil supplied from the surface openings of the recessed portion may flow toward the end surface at the opening side. The recessed portion may be provided as an axial groove.
To attain the above object, there is provided a motor comprising a bracket for holding a stator, a rotor making a relative rotation with the bracket, a rotor magnet generating a rotating magnetic field in corporation with the stator, and a dynamic bearing device supporting the rotation of the rotor, wherein the dynamic bearing device is constituted as described above. Such a motor may be appropriately applied in a spindle motor of an information-processing equipment such as a magnetic disk device (e.g., HDD or FDD), an optical disc device (e.g., CD-ROM or DVD-ROM), an optical magnetic device (e.g., MD or MO), a polygon scanner motor of a laser beam printer (LBP), or a small-sized motor of an electric equipment (e.g., an axial flow fan).
According to the present invention, it becomes possible to prevent the generation of the negative pressure in the housing and the cavitation due to the negative pressure. Thereby, it becomes possible to improve the rotational accuracy and the sealing performance against the lubricating oil in a dynamic bearing device and a motor having the same.